1. Field of the Invention
This invention relates to a photocurable and thermosetting resin composition and a photosensitive dry film formed from the composition, and more particularly relates to a photocurable and thermosetting resin composition which is suitable for the formation of a solder resist etc. in a household grade printed circuit board and an industrial grade printed circuit board and a photosensitive dry film which is suitable for various applications such as the production of circuit boards and the mounting of the electronic parts. This invention relates further to a method for forming a pattern by the use of the photocurable and thermosetting resin composition and the photosensitive dry film mentioned above.
2. Description of the Prior Art
In the household grade printed circuit boards and the industrial grade printed circuit boards, generally a solder resist is formed before supplying a solder during the step of mounting electronic parts for the purpose of preventing molten solder from adhering to irrelevant portions and protecting circuits. As a resin composition for this solder resist, a developing type solder resist composition which is used to form a pattern by a photolithographic method is widely adopted from the viewpoint of ensuring highly accurate formation of circuits with high density. Particularly, with due respect to the problem of environmental safety and from the viewpoint of cost, the solder resist composition of the alkali developing type which implements development with a dilute aqueous alkaline solution has come to play the leading role.
Such a solder resist composition usually contains a polyfunctional epoxy compound having two or more epoxy groups as a thermosetting component for the purpose of improving the resistance to soldering heat. However, since the polyfunctional epoxy compound exhibits high reactivity, the photocurable and thermosetting resin composition containing this compound exhibits unduly short shelf life (useful life) and is liable to gain in viscosity prior to being applied to a blank circuit board. As a result, it is possible to formulate it as the one-package type only with difficulty. Accordingly, the composition requires to be formulated as the two-package type consisting of a hardener solution containing a polyfunctional epoxy compound as a main component thereof and a main agent solution containing a photosensitive prepolymer as a main component thereof and a curing promotor etc. added thereto. This poses the problem in working properties because it is necessary to mix them immediately prior to use.
Further, when the photocurable and thermosetting resin composition containing the polyfunctional epoxy compound is formed into a dry film, the shelf life (useful life) of this film becomes short and the preservation in the refrigerated state at a temperature of not more than 0° C. is required. That is to say, it is deficient in shelf stability at room temperature. Further, this dry film poses the problem in working properties because increase of its temperature to room temperature is required prior to use thereof.
When the photocurable and thermosetting resin composition containing the polyfunctional epoxy compound is applied to a substrate, exposed to light, developed and then thermally cured, a cured film having excellent properties such as hardness owing to the cross-linking reaction of the polyfunctional epoxy compounds is obtained. However, when the cross-linking reaction unduly proceeds, there is the possibility of causing shrinkage of the coating film on curing and cracks therein.